What holds the layers together is the multi-layered board of Prepreg. The user will need to fuse them when they stack all the board’s layers by exposing it to high temperatures. They will then have the similarity between the overall board thicknesses with the Prepreg thickness. When the designers use the Prepreg on a multi-layered board, it must have specific characteristics. The new surface must not have excessive resin powder, cracks, foreign matter, defects, stains, or oil. It must have a smooth feel and look.
Engineers will probably have a more complex Prepreg if they have a complex PCB. As a result, they can only achieve the needed thickness for the board and Prepreg to use different types of Prepreg.
What are the differences of Prepreg and core?
PCB cores and laminates are similar and quite different. Your core is effectively one or more Prepreg laminates that are pressed, hardened, and cured with heat, and the core is plated with copper foil on each side. The Prepreg material is impregnated with a resin, where the resin is hardened but left uncured. Most manufacturers describe the prepreg as the glue that holds core materials together; when two cores are stacked on each side of a prepreg laminate, exposing the stack to heat causes the resin to begin bonding to the adjacent layers. The hardened resin slowly cures through cross linking, and its resulting material properties start to approach those of the core layers.
The resin material encases a glass weave, and the manufacturing process for this glass weave is very similar to that used to manufacture yarns. The glass weave can be quite tight (e.g., 7628 prepreg) or loose (e.g., 1080 prepreg), which is controlled with a loom during manufacturing. Any gaps and the overall homogeneity of the yarn will determine the electromagnetic properties, which is then responsible for dispersion, losses, and any fiber weave effects seen by signals in the board.
PCB core vs. prepreg materials can have somewhat different dielectric constants, depending on the resin content, type of resin, and glass weave. This can be a problem when designing boards that require very precise impedance matching as the effective dielectric constant seen by a signal on a track depends on the dielectric constants of the surrounding materials. Not all prepreg and core materials are compatible with each other, and core/prepreg stacks with very different dielectric constants make it difficult to predict exact dielectric constants and losses in an interconnect .
With any PCB core or prepreg material, creepage and leakage current is a concern at high voltage. Electromigration of copper and subsequent growth of conductive filaments are one reason for creepage specifications for FR4 materials. This problem, as well as a desire to increase glass transition and decomposition temperatures, motivated a switch to non-dicyandiamide (non-DICY) resins in FR4 cores and laminates. Phenolic resins provide higher decomposition and glass transition temperatures compared to DICY resins while also providing higher insulation resistance after full curing.